A chroma keyer processes an image, or a sequence of images for motion video, to identify pixels in the image within a particular range of colors. The range of colors is called the key. The closeness of the color of each pixel, defined either as chrominance or red, green and blue (RGB) values, determines an alpha value for that signal. The set of all alpha values for all the pixels in the image defines what is called an alpha matte. This alpha matte is then used to composite the image, usually a foreground image, with one or more other images, usually background images. Green screen and blue screen production techniques rely on such chroma key operations. Luma keyers also may be used to extract an alpha matte based on luminance instead of chrominance.
There are a variety of ways to define the key used by a chroma keyer or a luma keyer. A challenge in implementing any keyer is providing a way for an operator to understand how the key relates to the input image, and how changes to the key will change the resulting composited image.